A Semantic Web Approach To Everyday ... - Silvio Peroni's Website

< span class =" summary " > WWW 2010 Conference : < abbr class =" dtstart " title ="2010 -04 -26" > April 26 -< abbr class =" dtend " title ="2010 -10 -30" >30 , < span class =" location " > Raleigh , NC , USA . < table > < th > Name < th > Role < tr class =" vcard " > < td class =" fn " > Juliana Freire < td class =" role " > Program Committee Chair < tr class =" vcard " > < td class =" fn " > Michal Rappa < td class =" role " > Conference Chair < tr class =" vcard " > < td class =" fn " > Paul Jones < td class =" role " > Conference Chair < tr class =" vcard " > < td class =" fv " > Soumen Chakrabarti < td class =" role " > Program Committee Chair < body >

The table was enriched by additional data declaring it to be an event (a conference) and data about the event itself – the url, summary, location – and about four relevant individuals – with their names and roles within the conference – were associated where necessary to the actual content of the table. So far, so good, and no overlap to speak about. Things change dramatically, though, when the overall structure of the main hierarchy (the HTML table) is at odds with the intrinsic hierarchy of the microformat data, for instance if the people are organized in columns rather than rows. For instance: < table > Program Committee Chair < td > Conference Chair Conference Chair < td > Program Committee Chair Juliana Freire < td > Michael Rappa Paul Jones < td > Soumen Chakrabarti

Unfortunately, vcards are a hierarchy themselves, and if the hierarchy of vcards is organized differently from the hierarchy of the HTML table, as in the latter case, it is just impossible to define the four vcards for the four people organizing the conference. Thus in plain HTML the choice of one of two possible presentation models for the main hierarchy of content makes it trivial or completely impossible the existence of the second hierarchy. A possible and partial solution to express vcard hierarchies in the latter example is RDFa [Adida et al. 2008], a W3C recommendation. It describes a mechanism to embed RDF statements into HTML documents by using some HTML attributes (href, rel, rev, content) in combination with other ad hoc attributes (property, about, typeof) proposed in the recommendation itself. < table xmlns : vc =" http :// www . w3 . org /2006/ vcard / ns #" xmlns : my =" http :// www . essepuntato . it /2010/05/ myVCard #" > ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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< td about =" my : pcc " typeof =" vc : Role " > Program Committee Chair < td about =" my : cc " typeof =" vc : Role " > Conference Chair < td about =" my : cc " property =" vc : hasName " > Conference Chair < td about =" my : pcc " property =" vc : hasName " > Program Committee Chair < td about =" my : jf " rel =" vc : role " resource =" my : pcc " > < span property =" vc : fn " > Juliana Freire < td about =" my : mr " rel =" vc : role " resource =" my : cc " > < span property =" vc : fn " > Michael Rappa < td about =" my : pj " rel =" vc : role " resource =" my : cc " > < span property =" vc : fn " > Paul Jones < td about =" my : sc " rel =" vc : role " resource =" my : pcc " > < span property =" vc : fn " > Soumen Chakrabarti

Since all attributes live in the context of elements, the price to pay is that to assert everything we want to assert we often need to add some structurally unnecessary elements to the current markup hierarchy of a document, needed only to add the RDF statements (e.g., the span elements emphasized above). Even if that does not represent a significant problem for strict Semantic Web theorists, document architects and markup expert see this as a kludge and an inelegant compromise. 3.3. Wikis: no overlapping where some should be

The strength of wikis lies in their allowing users to modify content at any time. The mechanisms of change-tracking and rollback that are characteristics of all wikis, in fact, promote users’ contributions and make “malicious attacks” pointless in the long run, since previous versions can be easily restored. A number of tools exist that automatically discover “wiki vandalisms” and provide users with powerful interfaces to surf changes, diff subsequent versions and revert content. For instance, Huggle4 is an application dealing with vandalism in Wikipedia, based on a proxy architecture and .NET technologies. A straightforward interface allows users to access any version of a page, highlights contributions of a specific user and reverts the content to old versions. Even client-side tools – meant to be installed as browsers extensions or bookmarklets – exist to extend the rollback mechanisms of Wikipedia, giving users more flexibility and control over (vandalistic) changes. For instance, Lupin5 is a set of javascript scripts that check a wiki page against a list of forbidden terms so that authors can identify undesirable modifications and restore previous (good) versions without a continuous control over the full content of the page; yet again, Twinkle6 provides users powerful rollback functions and includes a full library of batch deletion functions, automatic reporting of vandals, and users notification functions. These tools are successful in highlighting vandalism and in identifying versions created by malicious users. However, although it is possible to revert the page to any previous version, all changes (even acceptable ones) that were subsequent to the malicious version cannot be automatically inherited by the restored page. For instance, let us consider versions V1, V2, and V3 of a wiki page, where versions V1 contains a baseline (acceptable) content, V2 is identified as a partial vandalism 4 http://en.wikipedia.org/wiki/Wikipedia:Huggle

5 http://en.wikipedia.org/wiki/User:Lupin/Anti-vandal 6 http://en.wikipedia.org/wiki/Wikipedia:Twinkle

tool

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and is agreed to be removed, but V3 contains (possibly, in a completely different section than the target of the malicious attack) relevant and useful content that was added before the vandalistic version V2 was declared as such. The task of removing the modifications of version V2 while maintaining (whatever is possible of) version V3 is a difficult, error-prone and time-consuming task if done manually, yet there is no tool we are aware of that automatically filters contributions from multiple versions and merges them into a new one (or, equivalently, removes only selected intermediate versions). Yet, it is possible to characterize the interdependencies between subsequent changes to a document in a theoretical way. In fact, literature has existed for a long time on exactly these themes (see for instance [Durand 2008] [Durand 1994]). Although a detailed discussion of abstract models of interconnected changes is out of scope for this paper – details and authoritative references can be found in the above mentioned works – what is relevant in this discussion is that they happen to assume a hierarchical form that is frequently at odds with the hierarchical structure of the content of the document, and as such most issues derive from the data structures in which content is stored and from the model for manipulating these structures. For instance, the fact that in the wiki perspective each version is an independent unit that shares no content (even unchanged content) with the other versions prevents considering multiple versions as overlapping structures coexisting on the same document. If we were able to make these hierarchies explicit we would be able to create models and tools to manipulate these documents in a more powerful way and to exploit the existing interconnections between the overlapping hierarchies. 4. INTRODUCTION TO EARMARK AND ITS SUPPORT FOR OVERLAPPING FEATURES

The presence of hidden overlapping structures – transparent to users but very difficult to handle by applications – is the common denominator for the scenarios described in the previous section. More than the overlap itself – that cannot be ignored as it does exist and carries important meanings – the problem we face lies in the way applications store such overlapping structures. In the XML world, in fact, the only way to do so is through the use of (complex) workarounds that force the multiple hierarchies into one hierarchy of a XML document. That makes very tricky to perform sophisticated analysis and searches. This section discusses a different approach to metamarkup, called EARMARK (Extremely Annotational RDF Markup) [Di Iorio et al. 2009] [Peroni & Vitali 2009] [Di Iorio et al. 2010] based on ontologies and Semantic Web technologies. The basic idea is to model EARMARK documents as collections of addressable text fragments, and to associate such text content with OWL assertions that describe structural features as well as semantic properties of (parts of) that content. As a result EARMARK allows not only documents with single hierarchies (as with XML) but also multiple overlapping hierarchies where the textual content within the markup items belongs to some hierarchies but not to others. Moreover EAMARK makes it possible to add semantic annotations to the content though assertions that may overlap with existing ones. One of the advantages of using EARMARK is the capability to access and query documents by using well-known and widely supported tools for Semantic Web. In fact, EARMARK assertions are simply RDF assertions, while EARMARK documents are modeled through OWL ontologies. The consequence is that query languages (such as SPARQL [Garlik & Seaborne 2010]) and actual existing tools, such as Jena7 and Pel7 http://jena.sourceforge.net

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let8 ) can be directly used to deal with even incredibly complicated overlapping structures. What is very difficult (or impossible) to do with traditional XML technologies becomes much easier with these technologies under the EARMARK approach. In the rest of this section we give a brief overview of the EARMARK model, while in Section 5 we describe how EARMARK can be used to deal with the issues presented earlier. The model itself is defined through an OWL document9 , summarized in Fig. 1, specifying classes and relationships. We distinguish between ghost classes - that define the general model - and shell classes - that are actually used to create EARMARK instances.

Fig. 1. An UML-like representation of the EARMARK ontology.

4.1. Ghost classes

The ghost classes describe three disjoint base concepts – docuverses, ranges and markup items – through three different and disjoint OWL classes10 . 8 http://pellet.owldl.com

9 http://www.essepuntato.it/2008/12/earmark 10 All

our OWL samples are presented using the Manchester Syntax [Horridge & Patel-Schneider 2009], which is one of the standard linearization syntaxes of OWL. The prefixes rdfs and xsd refer respectively to RDF Schema and XML Schema namespaces, while the empty prefix refers to the EARMARK ontology URI plus “#”. Moreover, we use the prefix c to indicate entities taken from an imported ontology made for the SWAN project [Ciccarese et al. 2008], available at http://swan.mindinformatics.org/spec/1.2/collections.html. ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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The textual content of an EARMARK document is conceptually separated from its annotations, and is referred to through the Docuverse class11 . The individuals of this class represent the object of discourse, i.e. all the containers of text of an EARMARK document. Class : Docuverse DatatypeProperty : hasContent Characteristics : Fun ct io na lP ro pe rt y Domain : Docuverse Range : rdfs : Literal

Any individual of the Docuverse class – commonly called a docuverse (lowercase to distinguish it from the class) – specifies its actual content with the property hasContent. We then define the class Range for any text lying between two locations of a docuverse. A range, i.e, an individual of the class Range, is defined by a starting and an ending location (any literal) of a specific docuverse through the properties begins, ends and refersTo respectively. Class : Range EquivalentTo : refersTo some Docuverse and begins some rdfs : Literal and ends some rdfs : Literal ObjectProperty : refersTo Characteristics : FunctionalProperty Domain : Range Range : Docuverse DatatypeProperty : begins Characteristics : Fun ct io na lP ro pe rt y Domain : Range Range : rdfs : Literal DatatypeProperty : ends Characteristics : Fun ct io na lP ro pe rt y Domain : Range Range : rdfs : Literal

There is no restriction on locations used for the begins and ends properties. That is very useful: it allows us to define ranges that follow or reverse the text order of the docuverse they refer to. For instance, the string “desserts” can be considered both in document order, with the begins location lower than the ends location or in the opposite one, forming “stressed”12 . Thus, the values of properties begins and ends define the way a range must be read. The class MarkupItem is the superclass defining artifacts to be interpreted as markup (such as elements and attributes). Class : MarkupItem SubClassOf : ( c : Set that c : element only ( Range or MarkupItem ) ) or ( c : Bag that c : item only ( c : itemContent only ( Range or MarkupItem ) ) DatatypeProperty : hasGener a l I d e n t i f i e r Characteristics : Fu nc ti onalProperty Domain : MarkupItem Range : xsd : string

11 This

class (and its name) is based on the concept introduced by Ted Nelson in his Xanadu Project [Nelson 1980] to refer to the collection of text fragments that can be interconnected to each other and transcluded into new documents. 12 http://en.wikipedia.org/wiki/Palindrome#Semordnilaps ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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DatatypeProperty : hasNamespace Characteristics : Fu nct io na lP ro pe rt y Domain : MarkupItem Range : xsd : anyURI Class : Class : Class : Class : Class : Class :

c : Collection c : Set SubClassOf : c : Collection c : Bag SubClassOf : c : Collection c : List SubClassOf : c : Bag c : Item c : ListItem SubClassOf : c : Item

ObjectProperty : c : element Domain : c : Collection ObjectProperty : c : item SubPropertyOf : c : element Domain : c : Bag Range : c : Item ObjectProperty : c : firstItem SubPropertyOf : c : item Domain : c : List ObjectProperty : c : itemContent Characteristics : FunctionalProperty Domain : c : Item Range : not c : Item ObjectProperty : c : nextItem Characteristics : FunctionalProperty Domain : c : ListItem Range : c : ListItem

A markupitem individual is a collection (c:Set, c:Bag or c:List, where the latter is a subclass of the second one and all of them are subclasses of c:Collection) of individuals belonging to the classes MarkupItem and Range. Through these collections it is possible to define a markup item as a set, a bag or a list of other markup items, using the properties element (for sets), item and itemContent (for bags and lists). Thus it becomes possible to define elements containing nested elements or text, or attributes containing values, as well as overlapping and complex structures. Note also that handling collections directly in OWL allows us to reason about content models for markup items, which would not be possible if we had used the corresponding constructs in RDF13 . A markupitem might also have a name, specified in the functional property hasGeneralIdentifier (recalling the SGML term to refer to the name of elements [Goldfarb 1990]), and a namespace specified using the functional property hasNamespace. Note that we can have anonymous markup items – as it is possible in LMNL [Tennison & Piez 2002] and GODDAG [Sperberg-McQueen & Huitfeldt 2004] – by simply asserting that the item belongs to the class of all those markupitems that do not have a general identifier (i.e., hasGeneralIdentifier exactly 0). 4.2. Shell classes

The ghost classes discussed so far give us an abstract picture of the EARMARK framework. We need to specialize our model, defining a concrete description of our classes. These new shell subclasses apply specific restrictions to the ghost classes. First of all, the class Docuverse is restricted to be either a StringDocuverse (the content is specified by a string) or an URIDocuverse (the actual content is located at the URI specified). Class : StringDocuverse DisjointWith : URIDocuverse SubClassOf : Docuverse , hasContent some xsd : string

13 http://hcklab.blogspot.com/2008/12/moving-towards-swan-collections.html

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Class : URIDocuverse SubClassOf : Docuverse , hasContent some xsd : anyURI

Depending on particular scenarios or on the kind of docuverse we are dealing with – it may be plain-text, XML, LaTeX, a picture, etc. – we need to use different kinds of ranges. Therefore, the class Range has three different subclasses: — PointerRange defines a range by counting characters. In that case, the value of the properties begins and ends must be a non-negative integer that identifies unambiguous positions in the character stream, remembering that the value 0 refers to the location immediately before the 1st character, the value 1 refers to the location after the 1st character and before the 2nd one, and so on. By using the hasKey OWL property, we also assert that two pointer ranges having equal docuverse, begin and end locations are the same range; — XPathRange defines a range considering the whole docuverse or its particular context specifiable through an XPath expression [Berglund et al. 2007] as value of the property hasXPathContext. Note that, by using these ranges, we implicitly admit that the docuverse it refers to must be an XML structure. Moreover, the properties begins and ends have to be applied on the string value obtained by juxtaposing all the text nodes identified by the XPath. By using the hasKey OWL property, we also assert that two xpath ranges having equal docuverse, XPath context, begin and end locations are the same range; — XPathPointerRange is an XPathRange in which the value of the properties begins and ends must be a non-negative integer that identifies unambiguous positions in the character stream as described for the class PointerRange. Class : PointerRange HasKey : refersTo begins ends SubClassOf : Range , begins some xsd : n on Ne ga ti ve In teg er and ends some xsd : n on Ne ga tiveInteger Class : XPathRange SubClassOf : Range EquivalentTo : hasXPathContext some rdfs : Literal HasKey : refersTo begins ends hasXPathContext Class : XPath Pointe rRange SubClassOf : XPathRange , begins some xsd : n on Ne ga ti ve In teg er and ends some xsd : n on Ne ga tiveInteger DatatypeProperty : hasXPathContext Characteristics : Fun ct io na lP ro pe rt y Domain : XPathRange Range : rdfs : Literal

MarkupItem is specialized in three disjointed sub-classes: Element, Attribute and Comment, that allow a more precise characterization of markup items. Class : Element SubClassOf : MarkupItem Class : Attribute SubClassOf : MarkupItem Class : Comment SubClassOf : MarkupItem DisjointedClasses : Element , Attribute , Comment 4.3. Range and markup item overlap

The presence of overlap in EARMAK is worth discussing more in detail. Different types of overlap exist – according to the subset of items involved – and different strategies are needed to detect them. In particular, there is a clear distinction between overlapping ranges and overlapping markup items. ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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By definition, overlapping ranges are two ranges that refer to the same docuverse and so that at least one of the locations of the first range is contained in the interval described by the locations of the second range (excluding its terminal points). Totally overlapping ranges have the locations of the first range completely contained in the interval of the second range or vice versa, while partially overlapping ranges have either exactly one location inside the interval and the other outside or identical terminal points in reversed roles. Thus, if we consider the following excerpt: Individual : r1 Types : PointerRange Facts : refersTo aDocuverse , begins "0"^^ xsd : n on Ne ga ti ve In te ger , ends "7"^^ xsd : n on Ne ga tiveInteger Individual : r2 Types : PointerRange Facts : refersTo aDocuverse , begins "4"^^ xsd : n on Ne ga ti ve In te ger , ends "9"^^ xsd : n on Ne ga tiveInteger

we can infer, through a reasoner such as Pellet, that these two ranges overlap by using the following rules: begins (x , b1 ) ^ ends (x , e1 ) ^ begins (y , b2 ) ^ ends (y , e2 ) ^ refersTo (x , d ) ^ refersTo (y , d ) ^ DifferentFrom (x , y ) ^ P -> overlapWith (x , y )

where P is one of: — — — —

lessThan(b1,e1) ˆ greaterThan(b2,b1) ˆ lessThan(b2,e1) lessThan(b1,e1) ˆ greaterThan(e2,b1) ˆ lessThan(e2,e1) lessThan(e1,b1) ˆ greaterThan(b2,e1) ˆ lessThan(b2,b1) lessThan(e1,b1) ˆ greaterThan(e2,e1) ˆ lessThan(e2,b1)

The case of overlapping markup items is slightly more complicated. We define that two markup items A and B overlap when at least one of the following sentences holds: (1) [overlap by range] A contains a range that overlaps with another range contained by B; (2) [overlap by content hierarchy] A and B contain at least a range in common; (3) [overlap by markup hierarchy] A and B contain at least a markup item in common. The three possible scenarios for such item overlap are summarized in Fig. 214 . The EARMARK ontology, in fact, is completed by another ontology15 that models all overlapping scenarios, either for ranges or markup items, and includes rules for inferring overlaps automatically, through a reasoner. 4.4. EARMARK as a standoff notation

If we ignore for a moment the semantic implications of using EARMARK and concentrate on its syntactical aspects only, it is easy to observe that EARMARK is nothing but yet another standoff notation, where the markup specifications point to, rather than contain, the relevant substructure and text fragments. 14 The

EARMARK documents describing these three overlapping scenarios and all the other ones presented in the following sections are available at http://www.essepuntato.it/2011/jasist/examples. 15 http://www.essepuntato.it/2011/05/overlapping ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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Fig. 2. Three EARMARK examples of overlapping between elements p.

Standoff notations, also known in literature as out-of-line notations [TEI Consortium 2005], are hardly new, but never really caught on for a number of reasons, most having to do with their perceived fragility under the circumstances of desynchronized modification to the text. In [Georg et al. 2010] and [Baski 2010] we can find a pair of recent and substantially complete analysis of their merits and demerits. In particular, according to , “standoff annotation has [...] quite a few disadvantages: (1) very difficult to read for humans (2) the information, although included, is difficult to access using generic methods (3) limited software support as standard parsing or editing software cannot be employed (4) standard document grammars can only be used for the level which contains both markup and textual data (5) new layers require a separate interpretation (6) layers, although separate, often depend on each other”16 . And yet, although EARMARK is in practice a standoff notation, it provides a number of workarounds to most of the above-mentioned issues. Firstly, since EARMARK is based on OWL and can be linearized in any of the large number of OWL caricaturisation syntaxes, it follows that 1) readability, 2) access and 3) software support for it are exactly those existing for well-known, widespread and important W3C standards such as RDF and OWL. Being able to employ common RDF and OWL tools such as Jena and SPARQL for EARMARK documents was in fact a major motivation for it. Issue 4 should be examined beyond the mere validation against document grammars and towards a general evaluation of the level of compliancy of the markup to some formally specified expectations. EARMARK documents, while being subject to no document grammar in the stricter XML sense, allow the specification of any number 16 In

order to individually address the issues, we edited the original bullets into a numbered list.

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of constraints, expressed either directly in OWL, or in SWRL [Horrocks et al. 2004] or even in SPARQL, that trigger or generate validity evaluations. In [Di Iorio et al. 2011] we tried to show that a large number of requirements, from hierarchical wellformedness in the XML sense, to validation requirements in terms of XML DTDs, to adherence to design patterns, can be expressed satisfactorily using these technologies. Item 5 regards the difficulty of standoff notations to provide inter-layer analysis on XML structures: separate interpretation of markup layers is easy, but identification and validation of overlapping situations is more complex: standoff markup is mainly composed of pointers to content, and does not have any direct way to determine overlap locations without some kind of pointer arithmetics to compute them. Validation of contexts allowing overlaps as describable using rabbit-duck grammars [SperbergMcQueen 2006] is also not trivial. In this regard EARMARK provides yet again a solution that does not require special tools: although OWL does not allow direct pointer arithmetics, SWRL on the contrary does, as shown in Section 4.3 where we described a batch of (SWRL-implementable) rules that do in fact determine overlapping locations on EARMARK documents with good efficiency. Finally, issue 6 refers to the fact that evolution of separate markup annotation layers need to take place synchronously, lest one of them become misaligned with the new state of the document. This is, in summary, the fragility of pointers, which can be considered the fundamental weakness of standoff, as well as of any notation that has markup separate from its content: if a modification occurs to the underlying (probably text-based) source, all standoff pointers that could not be updated at the same time of the change become outdated and possibly wrong. All standoff notations fall prey of this weakness, and there is no way to completely get rid of it. What is possible is to identify exactly what are the conditions under which such weakness acts, and see if there is a way to reduce the mere frequency of such events. In fact, in order for a standoff pointer to become outdated, several conditions must take place at the same time: — the standoff notation must be used as a storage format, rather than just as a processing format; — the source document must make sense even without the additional standoff markup (i.e., the standoff notation contains no information that is necessary for at least some types of document modifications); — the source document must be editable (and, in fact, must be edited) on its own; — the standoff pointers must rely on positions that change when the source is edited (e.g., character-based locations); — editing must be done in contexts and with tools that cannot or do not update the standoff pointers; — there must be no computable way to determine the modifications of the document (e.g. via a diff between the old and the new version). Of course, no standoff notation can rule out that these conditions occur on their documents. But it is worth pointing out that all six of them must occur, for standoff pointers to become outdated. EARMARK is not safe from these occurrences, either, but, at least for the use cases here described, one or more of these conditions simply do not apply: EARMARK is mostly used as a processing format, with no need to save it on disk (conversion from the source formats, e.g. MS Word, is described in Section 6 and does not require special storage), the data format described is either in a very specific format (such as MS Word or ODT) that in fact already does handle internally its data changes and requires the overlapping data exactly for this purpose, or is in fact the result of a diff action on successive versions of a document (as in the case of the wiki pages). Finally, EARMARK allows references to relatively stable fragment ids ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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of the documents (by using XPath ranges without specifying explicitly begin and end locations), rather than the extremely fragile character locations, further reducing the chances of outdated pointers. For this reason, without being able to completely rule out the possibility of standoff pointers to go wrong, we tend to consider it as a significantly little risk, at least for the use case here described. 4.5. Using OWL vs. RDF for standoff notations

EARMARK is strongly based on OWL 2 DL [W3C OWL Working Group 2009] to express multiple markup layers with possible overlapping ranges over the same content. OWL 2 DL is not the only possible choice for expressing standoff notations via Semantic Web technologies. In fact, RDF is another valid and effective model for dealing with the same issue, as shown in [Tummarello et al. 2005] by means of the open-source API RDF Textual Encoding Framework (RDFTef). This API was created to demonstrate a plausible way for handling overlapping markup within documents and identifying textual content of a document as a set of independent RDF resources that can be linked mutually and with other parent resources. Beside giving the possibility to define multiple structural markup hierarchies over the same text content, the use of RDF as the language for encoding markup allows to specify semantic data on textual content as well. But the real main advantage in using RDF is the possibility of using particular built-in resources appositely defined in the RDF syntax specification [Beckett 2004] for describing and dealing with different kinds of containers, either ordered (rdf:Seq) or unordered (rdf:Bag). Thus, RDF resources can be used to represent every printable element in the text – words, punctuation, characters, typographical symbols, and so on – while RDF containers can be used to combine such fragments and containers as well. Although RDF is not sufficient to define a formal vocabulary for structural markup – does a given resource represent an element, an attribute, a comment or a text node? In which way is a resource of a certain type related to others? – the specification of an RDFS [Brickley & Guha 2004] or of an OWL layer can successfully address this issues. Hybrid solutions obtained by mixing different models, even when they are built one upon another, may seem elegant but not necessarily the best choice. In fact, there exist well-known interoperability limits between OWL 2 DL and RDF that prevent the correct use of Semantic Web tools and technologies. In particular: — any markup document made using RDF containers (e.g. to describe what markup items contain and in which order) and OWL ontologies (e.g. to define classes of markup entities and their semantics) results in an set of axioms that end up outside of OWL DL and well within OWL Full, which limits the applicability of the most frequently used Semantic Web tools, that are usually built upon the (computationally-tractable) description logic underlying OWL 2 DL; — the individual analysis of each language may be not applicable when we have to check particular properties that lay between RDF and OWL layers. For example, verifying the validity of a markup document against a particular schema, which is one of the most common activities with markup, needs to be made to work with both markup item structures (that would be defined in RDF) and logical constraints about classes of markup items (e.g., elements only, attributes only, the element “p”, all the element of a particular namespace, etc., all of them definable in OWL). Being able to express everything we need directly in OWL addresses both issues quite straightforwardly. The well known absence of containers and sequences in OWL can be overcome by modeling classes in specific ways using specific design patterns such as [Ciccarese et al. 2008] and [Drummond et al. 2006]. ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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5. USING EARMARK

There are multiple applications for the EARMARK approach. The most interesting for this paper is its capability of dealing with overlapping structures in an elegant and straightforward manner. Under EARMARK such structures do not need to be specified through complex workarounds as with XML, but they are explicit and can be easily described and accessed. Sophisticated searches and content manipulations become very simple when using this ontological model. The goal of this section is to demonstrate the soundness and applicability of EARMARK by discussing how the use-cases presented in Section 3 are addressed. Notice that throughout the section we investigate multiple EARMARK data structures and documents, focussing on the feasibility and potentiality of such an ontological representation. 5.1. Looking for authorial changes in Office Documents

The discussion in Section 3.1 showed that both ODT (OpenOffice format) and OOXML (Microsoft Word format) use complex data structures to store overlaps generated by change-tracking functionalities. These structures make it very difficult to search and manipulate the content when using XML languages and tools. Even very simple edits generate a rather tangled set of overlapping elements. Let us recall the example mentioned in Section 3.1, where the user “John Smith” splits a single paragraph into two. The ODT representation is: < text : tracked - changes > < text : changed - region text : id =" S1 " > < text : insertion > < office : change - info > < dc : creator > John Smith < dc : date >2009 -10 -27 T18 :45:00 [... other changes ...] [... content ...] < text :p > The beginning and < text : change - start text : change - id =" S1 "/ > < text :p > also < text : change - end text : change - id =" S1 "/ > the end .

The OOXML representation , as shown in Section 3.1, is even more complex. In fact these formats exploit in large scale (tangled) fragmentation (OOXML), or milestones and stand-off markup (ODT) to deal with overlaps. EARMARK, on the other hand, stores overlapping data in a direct and streamlined manner that does not require tools to rebuild information from the twists of a treebased XML structure. The information is already available and expressed through consistent RDF and OWL statements. Fig. 3 graphically shows the corresponding EARMARK document. The original paragraph content and the new string “also” are now encoded as two docuverses over which the ranges r1, r2 and r3 are defined. The original paragraph is then composed of the (content of) ranges r1 and r2, while the paragraphs resulting after the (text and carriage return) insertion now comprise respectively range r1 and ranges r2, r3. Metadata about the author and the modification date are encoded as further RDF statements. Individual : doc1 Types : StringDocuverse Facts : " The beginning and the end " Individual : doc2 Types : StringDocuverse Facts : " also "

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Fig. 3. Encoding in EARMARK the ODT change-tracking example.

Individual : r1 Types : PointerRange Facts : refersTo doc1 , begin "0"^^ xsd : nonNegativeInteger , end "17"^^ xsd : nonNegati ve In te ge r Individual : r2 Types : PointerRange Facts : refersTo doc1 , begin "17"^^ xsd : nonNegativeInteger , end "25"^^ xsd : nonNegati ve In te ge r Individual : r3 Types : PointerRange , insJS Facts : refersTo doc2 , begin "0"^^ xsd : nonNegativeInteger , end "5"^^ xsd : nonNegative In te ge r Individual : p - b Types : Element Facts : firstItem p -b - i1 Individual : p -b - i1 Facts : c : itemContent r1 , nextItem p -b - i2 Individual : p -b - i2 Facts : c : itemContent r2 Individual : p - m Types : Element , insJS Facts : firstItem p -m - i1 Individual : p -m - i1 Facts : c : itemContent r3 , nextItem p -m - i2 Individual : p -m - i2 Facts : c : itemContent r2 Individual : insJS Types : Insertion Facts : dc : creator " John Smith " , dc : date "2009 -10 -27 T18 :45:00" Individual : p - t Types : Element Facts : firstItem p -t - i Individual : p -t - i Facts : c : itemContent r1

The advantages of streamlining overlaps becomes apparent if we consider tasks a little beyond the mere display. For instance, the query for “the textual content of all paragraphs inserted by John Smith” ends up rather entangled if we used XPath on the ODT structure. The process for finding that textual content needs to browse the current version of the document, look for all the text:change-start/text:change-end pairs that refer to an insertion made by John Smith involving the creation of a new paragraph (i.e., text:change-start is in a first paragraph while its pair, text:change- end, is in the following one), that are either currently present in the document body or hidden behind a subsequent deletion made by someone else. Once identified the paragraphs, we need to retrieve the content that originally was contained there, i.e., the text fragments that still are within those boundaries or that may have been deleted in ACM Journal Name, Vol. V, No. N, Article A, Publication date: January YYYY.

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subsequent versions. The following XPath represent an implementation of the above process: for $cr in (// text : changed - region ) , $date in ( $cr / text : insertion //( @office : chg - date - time | dc : date ) ) return $cr [.// text : insertion [(.// @office : chg - author = ’ John Smith ’ and count ( $cr // text : p ) = 2) or (.// dc : creator = ’ John Smith ’ and (// text : change - start [ @text : change - id = $cr / @text : id ]/ following :: text : p intersect // text : change - end [ @text : change - id = $cr / @text : id ]/( ancestor :: text : p ) ) ) ]]/ root () //(( text : change - start [ @text : change - id = $cr / @text : id ]/( following :: text : p //(( text () |( for $tc in ( text : change ) return // text : changed - region [ @text : id = $tc / @text : change - id and not ( text : insertion //( @office : chg - date - time | dc : date ) > $date ) ]// text : p [1]// text () ) ) except (( for $tc in ( text : change ) return $tc [ count (// text : changed - region [ @text : id = $tc / @text : change - id and not ( text : insertion //( @office : chg - date - time | dc : date ) > $date ) ]// text : p ) = 2]/ following :: text () ) union (// text : changed - region / text : deletion [.// dc : date $date ) ]// text : p [1]// text () ) ) except (( for $tc in ( following :: text : change ) return $tc [ count (// text : changed - region [ @text : id = $tc / @text : change - id and not ( text : insertion //( @office : chg - date - time | dc : date ) > $date ) ]// text : p ) = 2]/ following :: text () ) union (// text : changed - region / text : deletion [.// dc : date $date ]| ancestor :: w : del [ @w : date < tbody > < tr valign =" top " > 18 http://www.mediawiki.org 19 For

the sake of clarity we removed all markup irrelevant to our discussion.

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Angelo Di Iorio et al. Table I. All the versions of a wiki page modified by different authors.

Version

V1

V2

V3

V4

Author

151.61.3.122

Angelo Di Iorio

Silvio Peroni

Fabio Vitali

Content

Bob was farming carrots and tomatoes

Bob was farming carrots, tomatoes and beans

Bob was farming carrots, tomatoes and green beans. They were all tasteful .

Bob was farming carrots, tomatoes and green beans. [new paragraph] They were all tasteful.

< td class =" diff - otitle " > < div id =" mw - diff - otitle1 " > Revision as of 15:46 , 8 November 2009 < div id =" mw - diff - otitle2 " > 151.61.3.122 < td class =" diff - ntitle " > < div id =" mw - diff - ntitle1 " > Revision as of 15:47 , 8 November 2009 < div id =" mw - diff - ntitle2 " > Angelo Di Iorio < td class =" diff - marker " > - < td class =" diff - deletedline " >